Device and method for atomizing and comminuting liquid melts

ABSTRACT

The device for atomizing and comminuting liquid melts, including a slag tundish to whose outlet an expansion or cooling chamber is connected, whereby a propellant gas lance opens into the outlet, which propellant gas lance is surrounded by a tubular underflow weir immersed in the liquid slag, is dimensioned such that the width of the gap between the lower edge of the underflow weir and the tundish bottom is smaller than 20%, preferably smaller than 15%, of the clear width of the outlet, that the bottom of the tundish in the region between the lower edge of the overflow weir and the outlet is designed in a funnel-shaped manner, and that supercritical vapor is injected through the propellant gas lance to form an underexpanded free jet in the interior of the melt jet, the flow speed at the nozzle mouth of the lance being adjusted to sonic speed, thus ensuring a substantially enhanced comminution performance.

[0001] The invention relates to a device for atomizing and comminutingliquid melts, including a slag tundish to whose outlet an expansion orcooling chamber is connected, whereby a propellant gas lance opens intothe outlet, which propellant gas lance is surrounded by a tubularunderflow weir immersed in the liquid slag.

[0002] Devices of the initially defined kind may be used to atomize andgranulate slags, whereby a droplet size as small as possible is to berealized in order to cause the rapid cooling and hence vitrification ofthe slag and, at the same time, obtain a solidified material at afineness which renders subsequent grinding superfluous. This holds, inparticular, with the comminution and vitrification of slag melts which,on account of their latent hydraulic properties, may subsequently beused as cement substitutes or additives to cement mixtures. The tubularunderflow weir immersed in the liquid melt may be arranged so as to beadjustable in height in order to obtain a predetermined layer thicknessin the region of the outlet, wherein the propellant gas lance itselfmay, furthermore, be mounted in a height-adjustable manner so as toenable the adjustment of the respectivley optimum positioning in view ofthe ejection of the liquid melt in the form of a propellant gas jetjacket.

[0003] Departing from such an arrangement and a device of that type theinvention aims to provide a construction by which the desired particlesize can be substantially reduced, while, at the same time, improvingthe homogeneity of the grain sizes. The invention, in particular, aimsto improve the flow conditions in the outlet region with a view toavoiding fluttering of the film, splashing or an instable behavior ofthe melt and inducing additional shearing forces in the slag droplets tofurther enhance comminution. To solve this object, the device accordingto the invention essentially consists in that the width of the gapbetween the lower edge of the underflow weir and the tundish bottom issmaller than 20%, preferably smaller than 15%, of the clear width of theoutlet, that the bottom of the tundish in the region between the loweredge of the overflow weir and the outlet is designed in a funnel-shapedmanner, and that the propellant gas lance is configured for the use ofsupercritical vapor to form an underexpanded free jet in the interior ofthe melt jet. Thereby, conditions are created which enable the injectionof supercritical vapor through the propellant gas lance to form anunderexpanded free jet in the interior of the melt jet, wherebyparticularly critical flow conditions are created and the propellant gasstream is caused to emerge at a velocity substantially equalling sonicspeed. Taking into consideration the use of supercritical vapor and thefact that, as a result, an underexpanded jet is ejected, pressureimpacts in the range of Mach's nodes will subsequently occur withexpansion volumina lying between such Mach's nodes. Due to vibrationinterferences in the two-phase jet, shearing stresses are introducedinto the slag droplets, whereby the frequency is accordingly increasedat increasingly supercritical conditions, thus accordingly reducing thedistance between Mach's nodes in the axial direction of the propellantgas jet. The fact that an underexpanded jet is ejected at supercriticalconditions results in an expansion immediately upon emergence from thenozzle with the speed at the mouth of the nozzle corresponding exactlyto sonic speed, if the pre-pressure in the nozzle is at leastsupercritically higher than the pressure prevailing immediately uponemergence from the nozzle. By choosing the width of the gap between thelower edge of the underflow weir and the tundish bottom to be smallerthan 20% and, preferably, smaller than 15% than the clear width of theoutlet, it is ensured that a sufficiently central free space will beobtained within the running-off jet, which enables the appropriatelowering of the propellant gas lance to close to, or into, the outletopening. This is of importance, particularly because, bearing in mindthe special flow conditions brought about by a supercritical vapor or anunderexpanded free jet, the melt nozzle is subjected to high mechanicalstresses in the region of the outlet of the tundish, unless Mach'svibration nodes occur at a sufficiently large distance from the outletmouth. It is, thus, to be safeguarded that no Mach's nodes will occur inthe region of the mouth itself in order prevent excessive wear byerosion and cavitation phenomena in the region of the slag or meltoutlet. By designing the bottom of the tundish in a funnel-shaped mannerin the region between the lower edge of the underflow weir and theoutlet, it is ensured that the formation of annular beads as a functionof the respective viscosity of the melt will be prevented in order toprevent the separation of the flow shortly after its entry into thebottom outlet and avoid fluttering of the film as well as optionallysplashing and an instable behavior of the melt. In the optimum case, theinclination of the funnel is dimensioned such that the layer thicknessdefined by the distance of the lower edge of the underflow weir from thebottom of the tundish will be largely maintained, or else reduced, asfar as to the brink of the outlet.

[0004] In accordance with the invention, when keeping supercriticalconditions for the vapor within the propellant gas jet and ejecting anunderexpanded free jet, it is operated in a manner that the flow speedat the nozzle mouth of the lance exactly corresponds to sonic speed.

[0005] In order to reliably prevent the formation of beads in the regionof the slag outlet, as indicated above, the configuration advantageouslyis devised such that the angle of inclination of the funnel-shapedtundish bottom region is chosen to be smaller than 30°, preferablyapproximately 20°, relative to the cross sectional plane of the outlet.

[0006] In principle, it is of advantage if the tundish and the melt orslag on stock in the tundish are heated, to which end an inductiveheating is advantageously provided. In the context of the invention, theconfiguration in a particularly advantageous manner is devised such thatthe tundish comprises heating elements to heat with medium-frequencycurrent, and that at least the underflow weir is made of an electricallyconductive material such as, e.g., C, SiC or ZrO₂ and ZrO₂.MgO,respectively, whereby, when using a medium-frequency current as opposedto applying high-frequency heating, by which only the external region ofthe slag tundish becomes appropriately heatable due to the skin effect,it is feasible to heat also the underflow weir if the latter is made ofan electrically conductive material. Advantageously, the configurationis devised such that the outlet region of the tundish is made of SiC,Al₂O₃ or ZrO₂ or ZrO₂.MgO, respectively, with the tundish preferablybeing made of graphite or SiC. In this manner, it is feasible, whenusing a medium-frequency current, to ensure appropriate heating oncritical points and use the respective heat transmission onto the meltbath for fine temperature adjustment as well as to melt possiblyentrained solids particles.

[0007] According to the invention, the configuration advantageously isdevised such that the propellant gas lance is designed for a propellantvapor having a temperature of between 600° and 1250° C. at a pressureranging between 2 and 5 bars.

[0008] The ejection of an underexpanded free jet necessarily causes sucha free jet to exhibit a certain divergence. However, taking into accountthe supercritical ejection conditions, this divergence is supposed to beextremely low and, strictly speaking, substantially constitutes a freejet having a cylindrical envelope with a relatively low divergence. Sucha jet geometry exhibiting a relatively low divergence offers theadvantage that the nozzle mouth of the lance can be kept a largerdistance (S) from the outlet opening without the wear in the region ofthe outlet substantially increasing. Particularly favorable conditionsin terms of wear, of a slag outlet optionally designed as a separatecharging component in this respect advantageously may be achieved inthat the slag outlet of the tundish, following the funnel-shaped inlet,is designed to be hollow-conical or cylindrical over an axial lengthcorresponding to 0.6 to 1.1 times the clear diameter of the outletopening, the outlet opening advantageously widening conically followingthe hollow-conical or cylindrical region.

[0009] In order to optimize the rheologic properties of slags, inparticular of, for instance, typical blast furnace slags, the additionof fluxes such as, e.g., fluorspar (CaF₂) has proved to be extremelyadvantageous. The dynamic viscosity of a blast furnace slag having a CaOto SiO₂ ratio of about 1 and an Al₂O₃ content of about 9% by weight at1480° C. may, thus, be divided by half through the addition of, forinstance, 1.2% by weight of CaF₂, thus both saving the refractorymaterial and increasing droplet comminution. At the same time, anincrease in the early strength of mixed cements produced by using suchcomminuted products has been observed.

[0010] In the following, the invention will be explained in more detailby way of an exemplary embodiment of the device according to theinvention schematically illustrated in the drawing. Therein, FIG. 1illustrates a section through the tundish in the region of the slagoutlet and FIG. 2 shows an enlarged detailed illustration of a modifiedslag outlet.

[0011] FIG. 1 depicts a propellant-vapor gas lance 1 including anejection nozzle, which is mounted so as to be adjustable in height inthe sense of double arrow 2. The propellant-vapor gas lance isconcentrically surrounded by a tubular weir 3 made of an electricallyconductive material. This tubular weir 3 likewise is adjustable inheight in the sense of double arrow 4, in order to ensure an appropriatedistance of the lower edge of the tubular underflow weir relative to thebottom 5 of a tundish 6. The tundish 6 is closeable by a lid 7 andcomprises an outlet 8 whose surfaces facing the melt 9 are designed in afunnel-shaped manner. The angle of inclination α substantially amountsto 20° so as to ensure a laminar flow of the slag in the outlet regionat a given slag viscosity. The slag outlet 8 is designed as a separatecharging component, widening in a funnel-shaped manner on its externalside 10 facing away from the slag chamber, whereby the angle of conicityβ of this funnel-shaped enlargement is chosen to be larger than, orequal to, the angle of divergence γ of the free jet 11, which isillustrated in the drawing in an exaggerated manner. The propellant gasor propellant vapor—gas mixture leaves the lance 1 in a manner so as toform underexpanded jet, which results in the formation of Mach'svibration nodes 12 between expansion volumes 13. After its emergencefrom the lance 1, the free jet collides with the liquid slag jacket,thus causing induced shearing strains and hence further dropletcomminution.

[0012] The slag tundish is heated with a medium-frequency current, thepower source being denoted by 14 and the pertinent coil being denoted by15. The coil itself may be heated by the aid of cooling water, which issupplied through duct 16, so that heated cooling water may be drawn offthrough connection 17. Since inductive heating is realized by means ofmedium-frequency current, skin effects are avoided and it is feasible toheat also the tubular underflow weir 3, if the latter is made of anelectrically conductive material such as, for instance, carbon, siliconcarbide or zirconium. In the latter case, it is to be taken into accountthat zirconium exhibits the required electric conductivity likewise attemperatures of above 800° C.

[0013] When using a propellant vapor at a lance pressure ofapproximately 2.5 bars and a temperature of approximately 800° C., andhence supercritical conditions, rapid expansion and hence rapid coolingto temperatures in the range of 650° C. are caused immediately uponemergence from the lance at a flow velocity corresponding to sonicspeed. If a propellant vapor is used at a temperature of 600° C. and apressure of approximately 2.5 bars, the temperature will decrease toabout 475° C., whereby an ejection at sonic speed and, thus, thecreation of shearing forces suitable to effectively comminute the slagdroplets will be achieved also in this case.

[0014] FIG. 2 depicts the slag outlet 8 on an enlarged scale. As alreadymentioned, the angle of divergence γ represented in FIG. 1 does notcorrespond to the actual conditions prevailing at a supercritical andunderexpanded jet, whose envelope has nearly a cylindrical shape. As isapparent from FIG. 2, a substantially hollow-cylindrical region isprovided to follow the funnel-shaped configuration while enclosing anangle of inclination α, the axial length L of said hollow-cylindricalregion substantially corresponding to the diameter D of the clear widthof the outlet opening. Following this substantially hollow-cylindricalregion, the slag outlet opening again widens in a funnel-like manner asindicated by the conical walls 10.

1. A device for atomizing and comminuting liquid melts, including a slagtundish to whose outlet an expansion or cooling chamber is connected,whereby a propellant gas lance opens into the outlet, which propellantgas lance is surrounded by a tubular underflow weir immersed in theliquid slag, characterized in that the width of the gap between thelower edge of the underflow weir (3) and the tundish bottom (5) issmaller than 20%, preferably smaller than 15%, of the clear width (D) ofthe outlet (8), that the bottom (5) of the tundish (6) in the regionbetween the lower edge of the overflow weir (3) and the outlet (8) isdesigned in a funnel-shaped manner, and that the propellant gas lance(1) is configured for the use of supercritical vapor to form anunderexpanded free jet (11) in the interior of the melt jet, the flowspeed at the nozzle mouth of the lance (1) being adjusted to sonicspeed.
 2. A device according to claim 1, characterized in that the angleof inclination α of the funnel-shaped tundish bottom region (5) ischosen to be smaller than 30°, preferably approximately 20°, relative tothe cross sectional plane of the outlet (8).
 3. A device according toclaim 1 or 2, characterized in that the tundish (6) comprises heatingelements (15) to heat with medium-frequency current, and that at leastthe underflow weir (3) is made of an electrically conductive materialsuch as, e.g., C, SiC, ZrO₂ or ZrO₂.MgO.
 4. A device according to claim1, 2 or 3, characterized in that the outlet region (8) of the tundish(6) is made of SiC, Al₂O₃, ZrO₂ and/or ZrO₂.MgO.
 5. A device accordingto any one of claims 1 to 4, characterized in that the tundish (6) ismade of graphite or sic.
 6. A device according to any one of claims 1 to5, characterized in that the propellant gas lance (1) is designed for apropellant vapor having a temperature of between 600° and 1250° C. at apressure ranging between 2 and 5 bars.
 7. A device according to any oneof claims 1 to 6, characterized in that the slag outlet (8) of thetundish (6), following the funnel-shaped inlet, is designed to behollow-conical or cylindrical over an axial length corresponding to 0.6to 1.1 times the clear diameter (D) of the outlet (8).
 8. A deviceaccording to claim 7, characterized in that the outlet (8) widensconically following the hollow-conical or cylindrical region.
 9. Amethod for atomizing and comminuting liquid melts using a deviceaccording to any one of claims 1 to 8, characterized in that asupercritical vapor is injected through the propellant gas lance to forman underexpanded free jet in the interior of the melt jet with the flowspeed at the nozzle mouth of the lance being adjusted to sonic speed.10. A method according to claim 9, characterized in that a propellantvapor having a temperature of between 600° and 1250° C. at a pressureranging between 2 and 5 bars is used.
 11. A method according to claim 9or 10, characterized in that fluxes such as, e.g., CaF₂ are added toliquid slags to enhance their rheologic properties.